1. Field of the Invention
The invention relates to crosslinkable compositions which comprise silyl-terminated polymers, to processes for preparing them, to moldings produced from these compositions and to the use of the compositions for adhesively bonding workpieces.
2. Description of the Related Art
Polymer systems which possess reactive alkoxysilyl groups have been known for a long time. In the presence of atmospheric moisture, these alkoxysilane-terminated polymers are capable, even at room temperature, of condensing with one another, with elimination of the alkoxy groups. Depending on the level of alkoxysilane groups and on their construction, the products of the condensation are primarily long-chain polymers (thermoplastics), relatively wide-meshed, three-dimensional networks (elastomers) or else highly crosslinked systems (thermosets).
In line with the countless possibilities for the design of silane-terminated polymer systems of this kind, it is possible for not only the properties of the non-crosslinked polymers or of the polymer-containing mixtures (viscosity, melting point, solubilities, etc.) but also the properties of the fully crosslinked compositions (hardness, elasticity, tensile strength, elongation at break, heat resistance, etc.) to be tailored on a virtually custom basis. Correspondingly diverse, therefore, are the possibilities for use of such silane-terminated polymer systems as well. Thus, for example, they can be used for producing elastomers, sealants, adhesives, elastic adhesive systems, rigid and flexible foams, any of a very wide variety of coating systems, or for impression compounds. These products can be applied in any form, as for example by spreading, spraying, pouring, pressing, knifing, etc., depending on the composition of the formulations.
In addition to the curing of the compositions and the mechanical properties of the vulcanizate, a requirement, particularly in the case of applications in the adhesives and sealants segment, is for good adhesion to a host of different substrates, and good elastic properties. Formulations of silane-crosslinking polymers generally exhibit very good properties in these respects.
The adhesion profile is often enhanced or optimized by addition of organofunctional silanes as adhesion promoters. Silanes having primary amino groups in particular, such as 3-aminopropyltrimethoxysilane, provide a distinct improvement in adhesion properties, and hence this type of silane is present in virtually all adhesives and sealants based on silane-terminated polymers. The use of such silanes is known to the art and is described in various monographs or publications. In addition, there are also special, newly developed adhesion promoter silanes, as described in EP-A 997 469 or EP-A 1 216 263, although a combination of silanes, as shown in EP-A 1 179 571, is often conducive.
In addition to good adhesion, adhesives, and especially sealants, must also exhibit very good elasticity. A part is played here not only by the elongation, but also by the relaxation after elongation or compression. This quality is typically measured as compression set, creep behavior, or resilience behavior. For example, the ISO 11600 standard requires a resilience of more than 60% or even 70% for elastic sealants.
The elastic behavior is often determined by the formulation, but also by the nature of the silane-crosslinking base polymers. Organic silane-crosslinking polymers, especially those with difunctional end groups on the polymer, often exhibit inadequate resiliences. Here, it is the formulation that is critical for the properties. For example, U.S. Pat. No. 6,576,733 describes a way of improving the resilience by means of a special catalyst system which, however, contains tin. It is known, further, that the use of branched polymers produces an increase in the network density and hence an improvement in the elasticity. A disadvantage here, however, is the reduction in the chain lengths between two network nodes that accompanies branching, and that usually leads to a marked deterioration in mechanical properties, particularly the elongation at break, but also the tensile strength.
DE-A 102006022834 describes the use of aminoalkylalkoxysilanes in combination with epoxy-functional silanes for improving resilience. Disadvantages in this case are an increase in modulus and deterioration of adhesion.
One type of silane-terminated polymer of particular interest is notable for the separation of the reactive alkoxysilyl groups only by one methylene spacer from an adjacent heteroatom. These so-called α-alkoxysilylmethyl end groups possess particularly high reactivity with respect to atmospheric moisture. Corresponding polymers are described in WO 03/014226, for example. For sufficiently rapid curing, these polymers need only very small amounts of toxicologically critical tin catalysts or none at all, and are able on requirement to attain substantially higher curing rates. Accordingly the use of α-alkoxysilyl-terminated prepolymers of this kind is usually particularly desirable.
Nevertheless, elastomers which can be produced from this highly reactive α-silane-crosslinking polymer type, in comparison to elastomers formed from conventional silane-crosslinking polymers which cross-link via γ-alkoxysilylpropyl end groups, possess the disadvantage of a much lower resilience, which for many applications is inadequate, especially when the aminoalkylsilanes with primary amine groups, that are particularly preferred for the adhesion, are used.